Electrically conductive organic polymers have been of scientific and technological interest since the late 1970's. These relatively new materials exhibit the electronic and magnetic properties characteristic of metals while retaining the physical and mechanical properties associated with conventional organic polymers. Herein, we describe substituted and unsubstituted electrically conducting polyparaphenylene vinylenes, polyparaphenylenes, polyanilines, polythiophenes, polyazines, polyfuranes, polypyrroles, polyselenophenes, poly-p-phenylene sulfides, polyacetylenes, combinations thereof and blends thereof with other polymers and copolymers of the monomers thereof. These materials have a large number of potential applications in such areas such as electrostatic charge/discharge (ESC/ESD) protection, electromagnetic interference (EMI) shielding, resists, electroplating, corrosion protection of metals, and they can ultimately replace metal in such areas as solder, wiring, etc. For these applications to be successful, these materials need to have good environmental and thermal stability and to replace metal, high levels of electrical conductivity is needed.
These polymers are made electrically conducting by exposure to a dopant. The dopants are most commonly monomeric in nature, however, polymeric dopants have also been used. The dopants most commonly used, to date, are readily washed out from the polymer by water, ammonium hydroxide, certain solvents, etc. In addition, the dopants can diffuse out or can degrade upon heating. In the above situations, the conductivity of the polymer decreases and at the extreme case, the dopant is removed completely, rendering the polymer non-conducting.
Since the doped electrically conducting polymer can readily lose the dopant upon exposure to certain environmental conditions, polymers doped with, in particular monomeric dopants and some polymeric dopants, cannot be used in certain environments.
The polyaniline class of conducting polymers has been shown to be the most suitable of such materials for commercial applications. Great strides have been made in making the material quite processable and thus has allowed coatings to be developed and commercialized for numerous applications.
However, a great number of applications have not yet been realized because of certain limitations characteristic of the current polyaniline derivatives.
In many of the potential applications, the conducting polyaniline needs to be blended with suitable thermoset or thermoplastic resins that have the appropriate mechanical and physical properties required for a given application. The compositions are typically processed by processes such as injection or compression molding; these processes require high temperature. For example, polycarbonate which is most commonly used in the fabrication of computer housings, keyboards, etc. is processed at temperatures exceeding 200.degree. C.
At these temperatures the current polyaniline derivatives tend to lose significant conductivity due to evaporation or decomposition of the dopant molecules. To date, none of the conducting polyanilines will survive temperatures exceeding 200.degree. C. for any significant period of time without loss of some level of conductivity. Thus, polyaniline cannot be processed with polycarbonate to result in a blend which has sufficient conductivity for ESD and in particular EMI shielding applications.
The temperature at which the dopant molecule is lost depends on the particular nature of the dopant. The non-conducting form or the non-doped form of the polyaniline is thermally stable to temperatures exceeding 400.degree. C. FIG. 1 shows the thermogravimmetric analysis (TGA), weight loss versus temperature curve, for a polyaniline base (non-conducting form). No significant weight loss is observed to temperatures of 400.degree. C. Polyaniline doped with volatile acids such as hydrochloric acid (HCL), tend to lose significant conductivity at 100.degree. C. The use of organic acids such as sulfonic acids tend to give higher thermal stability. However, for the typical sulfonic acids commonly used, significant loss of conductivity occurs at 200.degree. C. FIG. 2 shows a TGA curve for Versicon.TM. of Allied Signal, a polyaniline doped with a typical sulfonic acid. As can be seen 39% weight loss occurs between 200 and 300.degree. C. accompanied by a significant loss in conductivity. It is therefore desirable to increase the thermal stability of the conducting polyaniline.
The environmental stability of the conducting polyaniline also needs to be improved. Although the conductivity of the polymer does not degrade in ambient conditions, it does degrade upon exposure to water and alkaline solutions and in certain cases upon exposure to solvents.
Both of the above limitations stem from the very nature of the doping mechanism of polyaniline. Polyaniline is converted into a conductor by reacting the non-conducting precursor form of the material with suitable dopants, most commonly protonic acids. This route has been extensively studied in that numerous derivatives and variations exist. However, the N--H bond is very labile and therefore, base and water can easily abstract the proton and render the material non-conducting. Also, the thermal stability of these doped forms is limited by the volatility/stability of the acid used as discussed above.
One variation of this doping mechanism has been to use organic, non-protonic acid dopants, such as methylating agents or acid chlorides etc. (1) which result in a covalent bond with the dopant. This does show some improvement in the stability of the conducting form of the polyaniline. However, further advances are still necessary.
Furthermore, a third limitation exhibited by the polyanilines, as well as some of the other conducting polymers, is their conductivity being on the low end of the metallic regime. In order for these materials to compete with metals in certain applications such as interconnect technologies, the conductivity of these materials need to be increased. The conductivity is governed by the mobility of the carriers-the mobility of the carriers along the chain as well as between chains. The intrachain mobility is governed by the chain conformation, degree of conjugation, and chain defects. The interchain mobility is determined by the polymer crystallinity, degree of order, interchain distance, and the presence or absence of any interchain interactions. It is generally the interchain mobility that tends to be more critical in limiting the conductivity as the carriers need to hop from one chain to another. Because there is no connecting path between chains, the hopping is a slow process. It is therefore desirable to enhance the interchain mobility of the carriers and to also improve the intrachain mobility in these polymers to achieve higher levels of conductivity.